1. Field of the Invention
The present invention relates to a technology with respect to an exposure apparatus that is used in transcription steps of lithographic steps for manufacturing highly-integrated semiconductor circuit elements.
2. Description of the Related Art
A semiconductor device or liquid crystal display device is manufactured by technique known as photolithography, in which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus used in this photolithography process has a mask stage that supports a mask and a substrate stage that supports a substrate, and it transfers a mask pattern onto a substrate via a projection optical system while sequentially moving the mask stage and the substrate stage.
In recent years, there has been a demand for higher resolution of the projection optical system in order to respond to the further advances in terms of higher integration of the device pattern. As the exposure wavelength to be used becomes shorter, the resolution of the projection optical system becomes higher. As the numerical aperture of the projection optical system becomes larger, the resolution of the projection optical system becomes higher. Therefore, the exposure wavelength which is used for the exposure apparatus is shortened year by year, and the numerical aperture of the projection optical system is increased as well. The exposure wavelength, which is dominantly used at present, is 248 nm of the KrF excimer laser. However, the exposure wavelength of 193 nm of the ArF excimer laser, which is shorter than the above, is also practically used in some situations. When the exposure is performed, the depth of focus (DOF) is also important in the same manner as the resolution. The resolution R and the depth of focus δ are represented by the following expressions respectively.R=k1·λ/NA  (1)δ=±k2·λ/NA  (2)
In the expressions, λ represents the exposure wavelength, NA represents the numerical aperture of the projection optical system, and k1 and k2 represent the process coefficients. According to the expressions (1) and (2), the following fact is appreciated. That is, when the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ is narrowed.
If the depth of focus δ is too narrow, it is difficult to match the substrate surface with respect to the image plane of the projection optical system. There is concern that the margin is insufficient during the exposure operation. Accordingly, the liquid immersion method has been suggested, which is disclosed, for example, in PCT International Publication No. WO99/49504 as a method for substantially shortening the exposure wavelength and widening the depth of focus. In this liquid immersion method, the space between the lower surface of the projection optical system and the substrate surface is filled with a liquid such as water or any organic solvent so that the resolution is improved and the depth of focus is magnified about n times by utilizing the fact that the wavelength of the exposure light beam in the liquid is 1/n as compared with that in the air (n represents the refractive index of the liquid, which is about 1.2 to 1.6 in ordinary cases).
With respect to the water and the organic solvent filled between the lower surface of the projection optical system and the substrate surface, temperature must be controlled strictly. This is because, if the temperature of the liquid varies, the refractive index of the liquid varies; thereby, the wavelength of the exposure light beam varies in the liquid; thus, a defect occurs due to the varied exposure light beam. More specifically, the temperature of the liquid must be controlled to be in a range of ±0.01° C. with respect to a predetermined temperature of the liquid, and the temperature-controlled liquid must be supplied onto the substrate continuously.
However, the temperature of the liquid may be affected by ambient temperature therearound if pipes which supply the liquid from a temperature control device (thermostat reservoir) to liquid-supplying-nozzles are long. Also, if the liquid supply is stopped when a wafer is exchanged, the temperature of the liquid existing in the pipes varies; therefore, there is a problem in that the exposure process cannot be restarted immediately even if the liquid supply is restarted because the temperature of the liquid in the liquid immersion region is not in the above temperature range.
In addition, there is possibility that contaminants may invade the liquid from ports of the pipes. In particular, if the exposure apparatus is stopped for a long time for purposes such as repair and inspection, there is possibility that bacteria will grow in the liquid existing in the pipes. Therefore, there is a problem with respect to not only micro-patterning but also operation of the exposure apparatus.